1. Dermatitis:
Dermatitis is an inflammatory reaction of the skin, and is the most prevalent skin disease. Often in acute skin inflammations, clinically, edematous erythema is presented initially, followed by erythematous lesions with papules and serous papules, then formation of vesicles, pustules, erosions, crusts and scales, followed by healing process. When a skin inflammation becomes chronic, thickened skin, lichenification and pigmentations are observed, and they are often associated with itching.
Dermatitis includes contact dermatitis, atopic dermatitis, seborrheic dermatitis, nummular eczema, psoriasis, stasis dermatitis, dyshidrotic eczema, asteatotic eczema, autosensitization eczema, etc.
Of these, atopic dermatitis is induced by hypersensitive reaction to a foreign protein antigen, a substance derived from other organisms which exists as house dust in the environment, as well as the involvement of various other non-specific stimulatory responses and specific allergic reactions. It is characterized by pruritic eczema over wide areas of the body accompanied by dryness and abnormal barrier function of the skin, and many patients have atopic diathesis. Atopic dermatitis is an intractable, chronic inflammatory disease, which relapses between remission and exacerbation. It has been known that delayed reactions associated with infiltration of eosinophils and lymphocytes, and production of various cytokines at the site of inflammation are involved in the onset and chronicity of atopic dermatitis.
2. Treatment of Dermatitis:
Current treatments for atopic dermatitis involve elimination of onset/exacerbation factors and skin care, in combination with pharmacotherapy appropriate for symptoms, while steroid external preparations are most commonly used for treating dermatitis. Recently, tacrolimus, an immunosuppressive drug, has also been used for treating atopic dermatitis.
Despite their dramatic clinical effectiveness, however, steroid external preparations are drugs that elicit numerous side effects. Steroid external preparations are not always satisfactory due to their side effects, including skin thinning, atrophy, so-called “moon face” which results from fat deposition in the face, skin flush, hirsutism, and skin striae, etc. In particular, since affected areas in the face and neck have higher absorbability of drugs than other areas, when a steroid external preparation is applied to the face, etc., they are more susceptible to steroid dermatitis such as steroid-induced rosacea. Moreover, further application of steroid external preparations to control the redness of the skin in steroid dermatitis means that the patients suffer from a vicious cycle of continuous steroid use. This explains why many patients avoid application of steroid drugs to the face and neck. In addition to these adverse side effects and concerns with regard to the use of steroid external preparations, there are further complications with the use of steroid drugs on patients with fairly sensitive skin, including infants, children, women and the patients with concomitant diseases, with whom eczema and dermatitis are found frequently. In addition, the repeated and long-term use of steroid external preparations, which is necessary due to the chronic nature of inflammation in atopic dermatitis developed with a background of atopic diathesis, may result in steroid resistance, in which the steroid external preparations become less effective against dermatitis. Furthermore, patients who withdraw from long-term use of steroid external preparations often suffer from what is known as a “rebound phenomenon” characterized by aggressive recurrence of a worsened symptoms than prior to the treatment.
Other concerns with long-term use of steroid external preparations are skin flush and desquamation of the skin over the whole body, which often occur with the comorbidity of hair loss and swollen lymph nodes. In such conditions of erythroderma posteczematosa, the patients′ social lives are severely disrupted.
Generally, steroids used for treating atopic dermatitis are classified as the “strongest”, “very strong”, “strong”, “medium” and “weak” in order of their efficacy, and the choice depends on factors such as the body region, severity of the rash, age and the period of use. For atopic dermatitis treatment using steroids, it is recommended that a steroid of a rank that is sufficiently strong to suppress the inflammatory symptoms of atopic dermatitis is prescribed initially, which is then replaced by progressively weaker ranks of steroids once the symptoms are controlled, allowing the earliest withdrawal from the steroid use.
The “strongest” rank steroids include: clobetasol propionate and diflorasone diacetate, and “very strong” steroids include mometasone furoate, betamethasone butyrate propionate, fluocinonide, betamethasone dipropionate, difluprednate, budesonide, amcinonide, diflucortolone valerate, and hydrocortisone butyrate propionate. In addition, the “strong” rank steroids includes: deprodone propionate, dexamethasone propionate, dexamethasone valerate, halcinonide, betamethasone valerate, beclomethasone propionate, fluocinolone acetonide, etc.; and “medium” rank steroids include: prednisolone valerate acetate, triamcinolone acetonide, flumethasone pivalate, alclometasone propionate, clobetasone butyrate, and hydrocortisone butyrate. The “weak” rank steroids include: predonisolone and hydrocortisone acetate.
Patients with chronic atopic dermatitis due to repeated recurrence are often found to have a history of use of multiple steroids belonging to either the “strongest” or “very strong” rank.
On the other hand, the alternative external preparation available, i.e., tacrolimus, is an immunosuppressive agent, and hence it is not applicable to children with 6 years old of age or younger, women who are or may be pregnant, and patients with renal disorders. The application of tacrolimus also significantly increases the risk of complication of herpes virus infection. This is thought to be due to the effect of the disturbance in the barrier function of the skin suffering from dryness, combined with the steroid-induced depression in the local immune functions. More seriously, incidence of malignant tumors has been recently reported in infants and children, making the safety of tacrolimus questionable. In addition, some patients find some of the local strong irritative side effects including burning sensation and itching, which inevitably occur upon external application of tacrolimus, unbearable.
For the reasons above, a safe, effective, adverse effect-free therapeutic agent for dermatitis, which is not only applicable without irritation to affected areas of higher absorbability of drugs with high density of hair follicles such as the face and the neck, but also safe to apply to patients having sensitive skins, such as females and children, is much hoped for.
3. Natriuretic Peptides:
Natriuretic peptides (NPs) are classified into three known families, named atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP); their well-known members are composed of 28, 32, and 22 amino acid residues, respectively.
(1) ANP and BNP:
ANP and BNP are synthesized mainly by the atria and the ventricles, respectively, and released from the heart into the whole body. It is thought that nearly 100% of the circulating ANP and BNP in the blood originate from the heart. These ANP and BNP are reported to be deeply involved in numerous diseases, including hypertension, cardiomegaly, cardiac failure, myocardial infarction, valvular heart disease, cardiac dysrhythmia, and pulmonary hypertension.
Human ANP is a peptide produced and released by atrial cardiocytes, and is composed of 28 amino acids, of which the 7th cysteine and the 23rd cysteine are bonded by a disulfide bond to form a ring structure. ANP has been shown to have diuretic effects in the kidneys and relaxes/dilates vascular smooth muscle cells in the blood vessels. In contrast, human BNP is a peptide produced and released by ventricular cells, and is composed of 32 amino acids, of which the 10th cysteine and the 26th cysteine are bonded by a disulfide bond to form a ring structure. BNP also possesses both diuretic and vasodilatory effects. BNP was originally isolated and identified in the porcine brain in Japan in 1988, and is also called B-type natriuretic peptide.
Both ANP and BNP bind to the receptor NPR-A (also called GC-A) having a guanylate cyclase domain, and exert their effects stated above, by stimulating the production of cGMP. In fact, secretion of ANP is stimulated in response to an increase in the atrial pressure by its distension in congestive heart failure, etc., and through its action stated above, ANP relieves the symptoms of congestive heart failure, etc. Likewise, BNP's release is stimulated during certain conditions including myocardial infarction, and BNP, through its action mentioned above, relieves the symptoms associated with myocardial infarction, etc. (Refer to non-patent literature 1). Although most of the circulating BNP derives from the ventricles, some BNP is released by the atria. In cardiac failure, the level of expression of both ANP and BNP increases to as much as 100 times more the normal level, but the increase of BNP expression is reported to be both greater and faster than that of ANP. While ANP (hANP) is marketed as a prescription drug for treating acute cardiac failure in Japan, BNP is clinically used in the United States.
(2) CNP:
CNP, which was once thought to function only as a brain peptide because it was first found in the brain, has now been clarified to exist in the periphery as well. In the vessel walls, in particular, CNP specific receptors were found to be abundant in smooth muscle cells, and CNP is produced by cells of the monocyte/macrophage lineage and the endothelial cells. For those reasons, CNP is speculated to function in the vascular walls as a local mediator involved in inhibition of growth of vascular smooth muscle cells. Its clinical application is currently being investigated for possible prevention of restenosis by CNP administration, which occurs with a certain frequency after percutaneous transluminal coronary angioplasty (PTCA) performed on patients with ischemic heart failure.
Recently it has been reported that intravenous administration of CNP remarkably improves cardiomegaly and fibrosis associated with myocardial infarction, and improves cardiac functions in animal experiments. Cardiac fibrosis is known to cause diastolic ventricular failure and cardiac dysrhythmia. Since CNP possesses a powerful action to suppress fibroblast proliferation, the potential of CNP as an anti-fibrotic medication for the heart is under investigation. Since CNP is a hormone naturally occurring in the body, there is only little concern of it having adverse side effects; accordingly clinical application of CNP as a therapeutic agent for arteriosclerotic diseases and heart diseases is expected. Here, examples of CNP include CNP-22 composed of 22 amino acids, and CNP-53 wherein 31 amino acid residues are attached to the N-terminal of CNP-22.
(3) Natriuretic Peptide Receptors:
Natriuretic peptide receptors are classified into three subtypes; NPR-A receptor (also called GC-A) and NPR-B receptor (also called GC-B) both of which contain a guanylate cyclase domain, and NPR-C receptor which lacks a guanylate cyclase domain. It is known that ANP can bind to NPR-A and NPR-C receptors, BNP can bind to NPR-A and NPR-C receptors, and CNP can bind to NPR-B and NPR-C receptors.
It is suggested that the activation of NPR-A receptors induces vasodilatory action, diuretic action, and cell growth inhibitory action, while NPR-B receptors are abundant in vascular smooth muscle cells and thought to be involved in the growth inhibition of vascular smooth muscle cells.
(4) Relationship Between Natriuretic Peptides and Immune System:
Historically, natriuretic peptide was first discovered as a peptide released from the atria, later named ANP, and its vasodilatory and diuretic actions gathered attention. BNP and CNP were then discovered as peptides similar to ANP. This historical background offers an explanation to why any attention to the relationship between natriuretic peptides and the immune system has been focused on those related to the cardiovascular system. CNP knock-out mice demonstrated impaired growth of cartilage resulting in a dwarfism-like phenotype (refer to Non-patent literature 2), which directed some interest to the relationship between arthritis and natriuretic peptides.
ANP is implicated in playing a role in arthritis and sepsis as it inhibits the release of inflammatory cytokines including tumor necrosis factor (TNF-α) and interleukin 1β (IL-1β) by macrophages (refer to Non-patent literature 3). This literature, however, does not mention ANP's relationship with the skin.
Similarly, the blood concentration of BNP has been reported to increase with the rejection response following heart transplant, and therefore it is suggested that it is associated with the immune regulation in the cardiovascular system (refer to Non-patent literature 4). However, this literature does not describe any connection between BNP and the skin.
Taking into account the observation that there is an increase in the blood concentration of BNP during the heart graft rejection, Kuroski de Bold et al. have investigated the immunoregulatory action of natriuretic peptides, and have demonstrated that both ANP and BNP inhibit the lymphocyte growth (refer to Non-patent literature 5). However, there is no connection between natriuretic peptides and the skin mentioned in this literature.
Chiurchiu et al. on the other hand have investigated the immunoregulatory action of BNP focusing on its association with the heart diseases and sepsis, and showed that BNP promotes the release by macrophages of pro-inflammatory cytokines such as arachidonic acid, prostaglandin E2 (PGE2), and leukotriene B4 (LTB4), and also promotes the release of anti-inflammatory cytokines including interleukin 10 (IL10). Thus, while BNP is indicated to have some action in the regulation of inflammatory responses, whether BNP acts overall to suppress or promote inflammatory responses remains inconclusive in the literature (refer to Non-patent literature 6). This literature also does not mention any connection between BNP and the skin.
Similarly, CNP is reported to be released by macrophages (refer to Non-patent literature 7), and while investigating the roles of CNP in cardiac ischemia and myocardial damage after reperfusion, Scotland et al. report that CNP suppresses platelet aggregation and lymphocyte migration (refer to Non-patent literature 8). The connection between CNP and the skin, however, is not described in this literature.
Likewise, Obata et al. examined the roles played by CNP in myocarditis using a rat myocarditis model generated by injecting pig myosin. They reported that continuous administration of CNP for 1 week had suppressed necrosis and inflammation of the cardiac tissues, while at the same time promoted the regeneration of blood vessels, thereby preventing functional loss of the heart (refer to Non-patent literature 9). Nevertheless, there is nothing in this literature to suggest a connection between CNP and the skin.
In addition, based on the observation that CNP knock-out mice show a dwarfism-like phenotype, attention has been paid to the potential connection between CNP and cartilage growth. Agoston et al. demonstrated that when incubated with Dexamethasone, the primary cultured chondrocytes extracted from the tibial bones of mouse embryos had significantly increased the expression of CNP genes (refer to Non-patent literature 10). This literature, however does not describe any connections between CNP and the skin.
It is evident that the connections between natriuretic peptides and the immune system have drawn increasing attention in recent years, but it is limited only to the inflammation of the cardiovascular system and arthritis, and the relationship between dermatitis or more specifically atopic dermatitis and natriuretic peptides has never been reported.
(5) Reports on the Application of Natriuretic Peptides:
Following are some examples of a number of applications of CNP, BNP and ANP.
Toshiko Koide and her colleagues have proposed a preparation for repair/regeneration of tissues and organs, comprising a composition that comprises ANP, BNP, CNP and urodilatin (P-Uro), and their precursors and derivatives, or combinations thereof as an active ingredient, and that may comprise pharmaceutically commonly-used diluents, excipients, fillers, and auxiliary agents (refer to Patent Literature 1).
However, specific examples of repair and regeneration of tissues and organs relate only to the regeneration of myocardiocytes, hypodermal tissue, hair, and improvement of cracked, rough skin due to wet work; they all correspond to ANP administration. There is no statement which implies therapeutic agents for treating skin disease or skin texture-improving agents by means of administration of CNP or BNP.
Masaharu Tanaka and his colleagues have proposed a C-type natriuretic peptide exhibiting a growth inhibitory action of vascular smooth muscle cells, as well as a growth inhibitory preparation for vascular smooth muscle cells containing such peptides as its active ingredient (refer to Patent Literature 2).
This, however, relates to the use of CNP in a growth inhibitory agent of vascular smooth muscle cells but does not imply application of CNP or BNP to therapeutic agents for dermatitis.
Katsuhiko Nakada and his colleagues proposed an eye drop for promoting lacrimal secretion or for treating keratoconjunctival disorder, containing as its active ingredient a natriuretic peptide, and they list ANP, BNP and CNP as examples of usable natriuretic peptide (refer to Patent Literature 3).
This, however, only relates to the application of the property of ANP, CNP and BNP to promote lacrimal secretion in an eye drop for treating keratoconjunctival disorder, and does not indicate the use of CNP or BNP in a therapeutic agent to treat dermatitis.
Kazuwa Nakao and his colleagues proposed a composition for increasing the body length containing a guanyl cyclase B (GC-B) activator as the active ingredient, which is to be administered to an individual without FGFR3 abnormality (refer to Patent Literature 4).
This indicates an application of CNP in a composition for increasing the body height based on the finding that the nose-anus length in transgenic mice which over-expressed CNP was larger than that in normal litters, but dose not imply the use of CNP or BNP in a therapeutic agent for dermatitis.
Kazuwa Nakao and his colleagues also proposed a prophylactic agent or therapeutic agent for inflammation of the joints containing a guanyl cyclase B (GC-B) activator such as CNP as an active ingredient (refer to Patent Literature 5).
However, this relates only to the application of CNP in a therapeutic agent or prophylactic agent for inflammation of the joints based on a study revealing that, compared to their litter mates, the articular cartilages grow thicker in the transgenic mice which over-express CNP, along with the observation that arthritis is repressed by the continuous administration of CNP to model animals of arthritis. Hence this does not imply the application of CNP or BNP in a therapeutic agent for dermatitis.
In addition, Masaharu Tanaka and his colleagues reported that CNP differs from ANP and BNP in the structure and function effects as stated below (refer to Patent Literature 2).
“At present, both ANP and BNP are thought to act as a hormone secreted by the heart into the blood, as well as a neurotransmission factor, and to play an important role in maintaining the amount of body fluid and homeostasis of blood pressure . . . . There are many unknown points in the physiological roles of CNP as a natriuretic peptide. Namely, since CNP has an amino acid primary sequence similar to that of ANP and BNP and shows a natriuretic action and a hypotensive action by in vivo administration, CNP was relegated to the natriuretic peptide family. However, because the natriuretic action and hypotensive action of CNP are significantly weaker than those of ANP and BNP (from 1/50 to 1/100) . . . , CNP has held a unique position in the natriuretic peptide family, and has been presumed to be playing a role different from the maintenance of amounts of body fluid and homeostasis of blood pressure . . . . Comparing the structure of CNP with that of ANP/BNP, CNP differs from ANP or BNP in the following points. Namely, the primary amino acid sequence of CNP differs from that of ANP or BNP at the exocyclic N-terminal domain; of the 17 amino acid residues in the endocyclic domain, 5 residues and 4 residues in CNP differ from those in ANP and BNP, respectively. In addition, the structure of the exocyclic C-terminal domain of CNP largely differs from that of ANP or BNP, and CNP does not have the tail structure which exists in ANP or BNP (in the cases of ANP and BNP, 5 amino acid residues and 6 amino acid resides, respectively, are attached to the C-terminal of the cyclic structure in ANP and BNP; this structure is called a tail structure for descriptive purposes). Thus-described structural differences between CNP and ANP/BNP are obviously involved in the manifestation of the above-mentioned characteristic pharmacological effects of CNP.”